Forced induction technical question.

[bashbarnard]

So ive come to the point im starting to think about upgrading the gt2052s i have which in my eyes has been an epic little turbo.

How to calculate spool time?
How to calculate efficiency?

I know people will say look at the compressor map. Thats all well and good but ill be honest with you guys. Looks like a map to me. No ones ever explained how to read it may as well be upside down and written in mandarin.
Also thinking about supercharging just because why not. Im sort of hoping ruans gonna come lay some science down for me at some point. All ive been doing so far is drawing from other peoples experience and my knowledge as a tech.

[highwayman306]

All ive been doing so far is drawing from other peoples experience and my knowledge as a tech.

I think that's what most of us have done matey i do hope someone helps you out with this, could be good for the 2.1 scene

[ADV_93]

https://www.youtube.com/watch?v=0Lv4yjiuJSU

[Ruan]

Spool time is actually very difficult to calculate... Basically you need to look at the A/R (if it's fixed geometry), the size of the turbine wheel, know the rotating mass, know how much exhaust gas velocity your engine is producing - it's a combination of all three, unfortunately I can't say any fixed mathematics... But as a general rule, the Turbine A/R and the size of the turbine are the major defining factors, of course, the bigger the compressor wheel and turbine wheel, predictably the weight of the rotating assembly increases the bigger wheels you have...

I think I did a post about turbocharger compressor maps back along and how to read them, they're really very simple:



That's a GT2052 48 trim, 0.51 Compressor A/R.

I'll try and explain what these maps are showing.

The axis up the left is Pressure Ratio - the easiest way to describe this is imagine the air going in the front is a ratio of 1.0, that is at atmospheric pressure (for arguments sake you pedantic knobbers) it's roughly 1bar of atmospheric pressure (14.5psi) - the more the turbo compresses, the more this pressure ratio increases - if we say the turbo is producing 1bar (14.5psi) of boost, we really mean, the turbo has doubled the pressure ratio to 2.0, that is the pressure is precisely double that of the inlet side of the turbo.

The bottom axis is airflow - this is where it gets a little more complex. This is the amount of air being put into your engine - you now need to know a little about your engine to be able to come up with this figure - you need to know the volumetric efficiency of the engine - that is if you have a 2100cc engine - 2100 * %VE = actual CCs for two rotations of the crank. This can change with RPM, so it's not feasible to give it one figure across the entire RPM (think about how a petrol gets "on cam" - Diesels do too, just not quite as severe). I've used a figure of about 0.85-0.88 for 8/12v diesels, the 2.1 12v might be slightly better than the 1.9 8v - going for a lower number will give you more headroom

You're probably now thinking "But this number is in CCs or litres" - or a volumetric measurement that determines the volume of your engine, the graph is in lb/min airflow. The reason lb/min is used is because the mass of a given amount of air is constant vs the volume when it's density changes due to temperature and/or pressure. To calculate the mass of the volume of air your engine is consuming, I tend to use https://www.rbracing-rsr.com/calcboost.html - which also takes into account horsepower etc and gives you a nice CFM and LB/Min reading, along with a predicted required boost for that power. You need to know what AFR you want to run and make up some numbers for BSFC (I use about 0.38 which seems about right). This calculator works both ways, so it'll tell you how much airflow you need for x horsepower, and you can also just adjust the numbers to see what horsepower you should be making with a given airflow and AFR.

The reading of the map in the middle is two parts - you'll see that the map has a "shape" around the outside, this is pretty much the operating "envelope" for the turbo -

The line up the left hand side of the map is your "surge" line - surge is simply the limit of how much pressure you can create with a given airflow - naturally the faster the turbo spins, the more pressure - but if you're limited by the amount of air your engine will consume, the pressure will increase, but the airflow can't, so the flow becomes unstable and you get that characteristic "barking" of the turbo, if you're ON the throttle when this happens, you'll hear a "chattering" or "chirping" noise - this is the flow becoming unstable and air doing funny things inside the compressor housing and stalling - it's incredibly damaging to the turbo.

The line on the right hand side of the map is the "choke" line - choking is when the turbo literally cannot move any more air at a given pressure, that is the limit to how much air the turbo will move, we tend not to ever run turbos close to choke, we usually never have enough exhaust flow to drive a turbo big enough to cause this, so it's not too much of a concern.

The "islands" i.e. the circular bits on the map are the efficiency islands - these are what determine how efficiently the turbo operates at a given pressure and airflow - you can see on this map the islands extend off most of the time, the peak efficiency is right in the middle, you can see the percentage efficiency next to each island.

The intersecting lines are showing shaft speed, so this one goes from 80,000rpm to 160,000rpm at the very top of the graph - run more boost than this, the shaft speed rises exponentially for not much gain. Naturally, there's a sweet spot, usually right in the middle where the turbo is working at it's most efficient.

For example - I'll do a very quick, rough calculation on your engine based upon plucking numbers out my asshole:

Your engine is 127.4cid, we'll say it's 85%VE, 0.380lb/hp.hr BSFC, 150*F IAT, 18.5 AFR to make sure it's burning clean, peak power at 4000RPM and a target horsepower of 175hp. That gives us:
Predicted Boost - 22.2psi
Mass Flow - 20.5lb/min

If you plot that on the compressor map - excuse the horrific paint skilz - using some crappy online editor:



You're now probably thinking that's very close to the end of the graph, and yes it is, at 175hp as an OEM application, the 2052s is probably pushing it, for us, we end up running them at actually a much lower AFR, much closer to the smoke limit, we're actually closer probably to 17:1 which gives us 18.8lb/min and 19.2psi boost:



But you can see you're still pushing it quite close to the limits of what that turbo will do - in an OEM application this wouldn't be acceptable, take it up the side of a mountain on a red hot day without much intercooler airflow, towing a trailer, you're likely to pop the turbo - but that's the joy of tuning, you're taking advantage of the headroom left by manufacturers... You could probably get 180-190hp out of it on a cold day and a good intercooler setup, which is just about what we've found and that's the numbers that are floating around the forum. I personally made 175hp @ 4300rpm with about 22psi boost on my 2052v which uses a similar compressor, but a slightly more better hotside - that coincides with the numbers shown here.

You can now plot each RPM point dependant on how much boost you want to run, annoyingly there used to be a really nice online compressor map plotter from Squirrel Performance, but the site is down atm . It can be quite difficult as you may not know precisely what RPM the turbo will spool at, but you have to kinda guess where the boost will come in based upon experience and knowing turbine A/R sizing. You'll quickly see that the turbochargers you "want" to be using are the much more modern, efficient units, because we need very high compressor performance, you'll find that an old T3/T4 compressor will produce the performance you want, but then you realise that it's bloody gigantic... This is a pain because although, yes, you're probably closer to the efficiency islands at full power, the turbo isn't going to spool for shit, so you never use the turbo at lower RPMs anyway and the compressor is often a bit big and it's anti-surge performance is crap, even if you did get it to spool, so it'll just surge instead. This is why it's a bit of a tradeoff going for a smaller, more efficient unit, but unless you're purely using the car for going up the drag strip, you might as well have a good spool time and sacrifice the efficiency at the top of the map.

Hope this kinda makes sense?

[bashbarnard]

Thanks everyone. A help as always ill read this in more depth probably tomorrow when im not hungover and tierd.

[Ruan]

I'm assuming this was a bit in depth and/or boring!!

[bashbarnard]

Nope. Absolutely perfect and exactly what i was looking for man. All info in one place. It helps and i do honestly appreciate it.